Conventional power supply circuits including a type equipped with a buck-boost DC-DC converter used for stepping-up conversion and stepping-down conversion of direct-current (DC) voltages.
FIG. 7 is a circuit diagram of conventional power supply circuit 500 disclosed in PTL 1. Power supply circuit 500 includes input terminal 103 connected to commercial power supply 101. Diode bridge 105 is connected to input terminal 103. Input capacitor 107 is connected in parallel with diode bridge 105 in a subsequent stage. In addition, a buck-boost DC-DC converter is connected in a subsequent stage of diode bridge 105.
The buck-boost DC-DC converter includes inductor 109, high-side switching device 111, low-side switching device 113, diode 115, diode 117, and output capacitor 119. Both ends of output capacitor 119 function as output terminals 121 of power supply circuit 500. Load 123 is connected to output terminal 121.
Power supply circuit 500 further includes controller 125, current detector 127, high-side driver IC 129, and a bootstrap circuit. The bootstrap circuit includes bootstrap capacitor 131 and diode 133.
DC drive voltage Vcc is supplied to controller 125. Controller 125 generates a switching control signal that turns on and off high-side switching device 111 and low-side switching device 113 to match an output current value with a target current value.
In the bootstrap circuit, bootstrap capacitor 131 is charged with the DC drive voltage Vcc when both high-side switching device 111 and low-side switching device 113 are turned off. This electric charge is used to raise a ground level of the switching control signal of high-side switching device 111, and secure a drive voltage necessary for turning on high-side switching device 111.
In this buck-boost DC-DC converter of power supply circuit 500, the drive voltage necessary to turn on high-side switching device 111 can be secured by raising the ground level with the electric charge of bootstrap capacitor 131. A characteristic of electric current IL in inductor 109 changes with lapse of time when bootstrap capacitor 131 is charged since the electric current IL increases by an amount for charging. FIG. 8 shows this condition.
FIG. 8 illustrates waveforms of voltages and electric currents in voltage step-up operation of power supply circuit 500. FIG. 8 shows voltage VQ1 supplied to a gate of high-side switching device 111, voltage VQ2 supplied to a gate of low-side switching device 113, current IQ1 that flows between a drain and a source of high-side switching device 111, current IQ2 that flows between a drain and a source of low-side switching device 113, and current IL that flows through inductor 109. In FIG. 8, the vertical axis represents voltage or current, and the horizontal axis represents time.
High-side switching device 111 is turned on when voltage VQ1 is at value Vhi1, and is turned off when voltage VQ1 is at level Vlow1. Low-side switching device 113 is turned on when voltage VQ2 is at value Vhi2, and is turned off when voltage VQ2 is at level Vlow2.
During a boost operation, fundamentally, controller 125 switches the level of voltage VQ2 between levels Vhi2 and Vlow2 alternately at a predetermined period to turning on and off low-side switching device 113 periodically at the predetermined period while maintains voltage VQ1 at voltage level Vhi1 to continuously turn on high-side switching device 111. However, in order to charge bootstrap capacitor 131, controller 125 switches voltage VQ1 to level Vlow1 to turn off high-side switching device 111 only for a duration Tfoff1 in a duration Tfoff2 for which low-side switching device 113 is turned off, and maintains the voltage VQ1 at level Vhi1 for a duration outside of duration Tfoff1 to continuously turn on high-side switching device 111. This operation causes an electric current to flow from inductor 109 to bootstrap capacitor 131 and charge bootstrap capacitor 131. As a result, electric current IQ1 flows between the drain and the source of high-side switching device 111, and electric current IQ2 flows between the drain and the source of low-side switching device 113, as shown in FIG. 8. Accordingly, electric current IL flows to inductor 109 as shown in FIG. 8.
As illustrated in FIG. 8, a declining slope of electric current IL in the duration Tfoff1 for which voltage VQ1 of high-side switching device 111 is reduced to level Vlow1 becomes larger than slopes of electric current IL before and after the duration Tfoff1. This is because electric current IL that flows through inductor 109 increases by an amount of the current used to charge bootstrap capacitor 131. Therefore, even though the waveform of electric current IL does not become a precise triangular wave, a peak value of electric current IL is constant since bootstrap capacitor 131 is charged every duration Tfoff2 for which low-side switching device 113 is turned off, hence stabilizing the boost operation of the DC-DC converter.
However, in the case that bootstrap capacitor 131 is charged less frequently, that is, when bootstrap capacitor 131 is charged once per plural on-off periods of low-side switching device 113, a ripple current may appears in an output current of power supply circuit 500.